Multiple GPUs one expensive GPU, which gpu to buy for real time processing (not training)
Here's a link to some benchmarks that should give you some insights.

In my experience (I've used systems with both 1080s and V100s) I've found that as of about a year ago, a lot of the common tools couldn't use the V100s well.  Until we started doing some manual optimization, the 1080s were comparable if not better on common tasks.  Of course, once we put in the time to optimize, recompile drivers, etc., the V100's really came to life and outperformed everything else by several times.